1. Field of the Invention
The present invention relates to a product containing a glass substrate supporting at least one thin conductive layer of a metallic oxide and possessing low emissivity, low resistivity, and transparency. The present invention further relates to a process for obtaining said product by application of pyrolysis techniques on organometallic compounds, and to applications of this product.
2. Discussion of the Background
A glass substrate supporting at least one thin conductive layer of a metallic oxide and possessing low emissivity, low resistivity, and transparency may be used in many applications. For example, a glass substrate coated with a low emissivity layer can be used to make a window. By increasing the far infrared reflection coefficient on the side of the window facing the inside of the room, it is possible to reduce energy losses through the window resulting from the leakage of radiation from inside the room to the outside. Thus, persons in the room may remain comfortable during the winter at a lower energy cost. Furthermore, the efficiency of an insulating double window may be increased by combining a transparent substrate supporting a low emissivity layer with another transparent substrate and separating the substrates by means of a gas section, and positioning the low emissivity layer on the third surface, counting from the outer surface.
A transparent substrate supporting a low resistivity layer may be used as a windshield or heated rear window in an automobile by providing current feeds. In addition, such a product can be used as a transparent electrode in optoelectronic devices.
The thin layers exhibiting these properties are, for example, layers of indium oxide doped with tin (ITO), zinc oxide doped with aluminum, indium, or fluorine, or tin oxide doped with fluorine. The metallic oxide layers may be deposited on a glass substrate by several different procedures. For example, by vacuum thermal evaporation, by cathodic spraying, by application of a magnetron, and by pyrolysis of organometallic compounds sprayed in liquid, solid, or gaseous form on the surface of the glass substrate and heated to a temperature lower than the melting point of the glass substrate. The organometallic compounds placed in contact with a heated surface decompose while oxidizing and form a metallic oxide layer. Pyrolysis techniques have been developed to such a state that the layer may be deposited on the glass substrate in a line, such as a float line, while the glass is transported at high speeds. While the glass still exists as a continuous strip transported at a given speed and temperature, "precursor" organometallic products are deposited using one or several nozzles as the glass leaves the float bath. Sometimes the glass leaving the float bath reaches speeds of 20 m/minutes.
It is believed the conductivity and low emissivity properties of these layers are attributable, at least partially, from oxygen gaps in the layers. However, if the deposition is effected by pyrolysis, the glass continues on the production line and passes through a reheating drying frame which relaxes the stresses in the glass by controlling the decrease in the glass temperature. When the metallic oxide layer comes into contact with the surrounding air during these operations, it tends to oxidize appreciably. Therefore, it is necessary to effect a reduction annealing operation to maintain the oxygen gaps in the layers. The reduction annealing operation consists of removing the substrate containing layer from the production line and allowing it to remain in a chamber heated to approximately 450.degree. C. under a reducing atmosphere for a determinate period of time. The additional step poses numerous difficulties. It is considered a recovery operation, thereby increasing the cost of production by requiring additional equipment and a reheating of the glass. The reduction annealing operation prohibits thermal treatments such as bending and tempering performed before or after the annealing step. For example, if air-hardening is carried out before the reduction annealing operation, the high temperature required by the reduction annealing operation could destroy, or at least lessen, the effects of the glass-tempering procedure. However, if tempering is performed after the reduction annealing operation, air-hardening which normally requires temperatures of at least 650.degree. C. could reoxidize the layer. Therefore, the discovery of a method for thermally treating a glass substrate supporting at least one thin conductive layer of a metallic oxide without the necessity of performing a reduction annealing operation to preserve the high performance of the layer would be quite useful.
Furthermore, metal oxide layers on a glass substrate, in particular conductive low emissivity layers having interferential thickness, exhibit a color under reflection which is dictated by the thickness selected. For example, an ITO layer exhibits a blue color under reflection when the thickness is 180 nm and a green color under reflection when the thickness is 360 nm. A color is assessed using two parameters. The length of the dominant wave indicates the tone of the color. The purity, expressed as a percentage, indicates the intensity of that tone. If the purity is low, the color is not intense. If the purity is high, the color is very vivid and can be reproduced using monochromatic light.
Because the tone of the color and/or the purity of the color of a substrate under reflection may not be the tone and/or purity desired, or may even be unacceptable for a planned use, especially in the automotive industry where strict aesthetic constraints apply, it is therefore necessary to be able to control the color of a substrate under reflection. For example, by incorporating a layer having a specific composition and thickness, it is possible to obtain a quasi-neutral color under reflection. In addition, slight variations in the thickness of a layer may induce non-aesthetic iridescence.
To reduce interferential coloring under reflection, it has been suggested to deposit at least one intermediate layer on the substrate before depositing the conductive layer. This intermediate layer is chosen so as to possess a predetermined refraction index and geometric thickness, so that the overall color under reflection incorporating these two layers approaches neutrality.
Conventional layers possessing a suitable refraction index include metallic oxide- and metallic nitride-based layers, and mixtures thereof. For example, aluminum oxide may be combined with other oxides such as SnO.sub.2, ZnO, TiO.sub.2, etc., and silicon oxycarbide or oxynitride. The intermediate layer may reduce the coloring under reflection, but may entail manufacturing constraints.